The present application relates to the suppression of prelasing in an electro-optic Q-switch laser and especially to the use of a saturable absorber placed in the laser cavity to prevent prelasing.
In a passive Q-switch, a material of non-linear absorption characteristics is inserted into a laser cavity. At low power levels, the material has high absorption blocking one of the mirrors from the laser medium. When the pump energy source is pulsed, the amount of light in the laser cavity builds up, eventually reaching a level where the cavity Q increases and a Q-switched pulse is produced.
Prelasing is an undesirable condition in Q-switch lasers in which an output pulse is emitted prematurely. It is frequently caused by the degradation of the laser system contrast ratio due to optical stress, low quality polarizers, misalignment, or charge build-up on the Q-switch crystal. A variety of techniques have been used to reduce or eliminate prelasing including maintaining the high contrast level of the laser system with delicate bonding techniques used to minimize optical stress and using stringent requirements in connection with polarizing components and alignment procedures. Radioactive ionizing sources have also been employed to prevent static charged build-up on a LiNbO.sub.3 Q-switch crystal in low humidity and in changing temperature environments.
In one prior U.S. Pat. No. 5,355,383 to Lockard, a method and apparatus for the detection and control of prelasing in a Q-switch laser detects prelasing in the Q-switch laser and terminates the laser operation upon the detection. A detector senses the presence of light beyond a Q-switch and generates an appropriate electrical signal. A comparison stage circuit compares the detected signal with an established threshold value indicative of prelasing and generates a trigger signal if the detector signal exceeds the threshold value. In another prior U.S. patent to Lam et al., U.S. Pat. No. 5,005,176, a method and apparatus for Q-switching a laser uses a multiple quantum well or MQW integrated into a laser cavity to implement an active Q-switch device. The MQW initially absorbs and stores energy to prevent the device from lasing. In response to an applied electrical field, the MQW experiences a sudden charged carrier population inversion and emits a strong short duration pulse having a directionality coincident with that of the beam within the lasing cavity.
The present invention is directed towards preventing an electro-optic Q-switch laser from prelasing and the premature output of the laser energy and positions a saturable absorber in the laser cavity and aligned with a laser beam by reducing the system sensitivity to stress, alignment and polarizing component quality. The low transmittance in the absorber prevents prelasing and, after the Q-switch turn on of the laser, the absorber saturates into a high transmittance component allowing proper lasing with low loss.